Motor vehicles, such as, for example, hybrid vehicles, use multiple propulsion systems to provide motive power. This most commonly refers to gasoline-electric hybrid vehicles, which use gasoline (petrol) to power internal-combustion engines (ICEs), and electric batteries to power electric motors. These hybrid vehicles recharge their batteries by capturing kinetic energy via regenerative braking. When cruising or idling, some of the output of the combustion engine is fed to a generator (typically the electric motor(s) running in generator mode), which produces electricity to charge the batteries. This contrasts with all-electric cars which use batteries charged by an external source such as the grid, or a range extending trailer. Nearly all hybrid vehicles still require gasoline as their sole fuel source though diesel or other fuels such as ethanol or plant based oils have also seen occasional use.
Batteries and cells are important energy storage devices well known in the art. The batteries and cells typically comprise electrodes and an ion conducting electrolyte positioned therebetween. Battery packs that contain lithium ion batteries are increasingly popular for automotive applications and various commercial electronic devices because they are rechargeable and have no memory effect. Storing and operating the lithium ion battery at an optimal operating temperature is very important to allow the battery to maintain a charge for an extended period of time.
Due to the characteristics of the lithium ion batteries, the battery pack operates within an ambient temperature range of −20° C. to 60° C. However, even when operating within this temperature range, the battery pack may begin to lose its capacity or ability to change or discharge should the ambient temperature fall below 0° C. Depending on the ambient temperature, the life cycle capacity or charge/discharge capability of the battery may be greatly reduced as the temperature strays from 0° C. Nonetheless, it may be unavoidable that the lithium ion battery be used where the ambient temperature falls outside the temperature range.
Alluding to the above, significant temperature variances can occur from one cell to the next, which is detrimental to performance of the battery pack. To promote long life of the entire battery pack, the cells must be below a desired threshold temperature. To promote peak performance, the differential temperature between the cells in the battery pack should be minimized. However, depending on the thermal path to ambient, different cells will reach different temperatures during charge and discharge operations. Accordingly, if one cell is at an increased temperature with respect to the other cells, its charge or discharge efficiency will be different, and, therefore, it may charge or discharge faster than the other cells. This leads to a decline in the performance of the entire pack.
Therefore, there remains an opportunity to improve upon prior packs of lithium batteries to increase the ambient temperature range at which the lithium battery operates and to provide a new battery pack with improved packaging characteristics. Likewise, there remains an opportunity to maintain the battery pack at the optimal operating temperature to ensure the longest possible life cycle, rated capacity, and nominal charge and discharge rates.